1. Field of the Invention
The present invention relates to a magnesium diaphragm having an anodic oxide coating on a surface thereof, a method of manufacturing the same, and a speaker apparatus.
2. Description of Related Art
Diaphragms used for speakers known in the art are magnesium diaphragms made of metal whose main component is magnesium. This is because diaphragms made of metal having a relatively large specific gravity, such as aluminum or titanium, have a small internal loss. However, magnesium is more readily rusted than aluminum, titanium and the like. Therefore, a rust-preventive treatment is applied to the surface of the magnesium diaphragms. (See, for example, Reference 1: Japanese Patent Laid-Open Publication No. 2002-369284, page 3, right column to page 4, left column, and Reference 2: Japanese Patent Laid-Open Publication No. Hei 11-236698, page 3, left column to page 5, right column.)
A magnesium diaphragm disclosed in Reference 1 has a layered-structure surface composed of an epoxy resin-based primer layer and an acrylic resin-based top layer. These layers are formed in two steps. First, the primer layer is formed on the magnesium diaphragm by means of baking finish. Then, an acrylic resin-based top layer is formed on the primer layer, also by means of backing finish. Such layered structure enhances not only the anti-corrosiveness of the magnesium diaphragm, but also the decorative value of the diaphragm.
However, this magnesium diaphragm disclosed in Reference 1 is thick and heavy, because the layers are formed by baking finish. Being thick and heavy, the magnesium diaphragm may not exhibit so good characteristics as is desired.
A magnesium diaphragm disclosed in Reference 2 has a layered-structure surface composed of an anodic oxide coating and an electrodeposition coating. Theses coatings are formed in two steps. First, the magnesium diaphragm is subjected to anodic oxidation, forming an anodic oxide coating on the surface. Next, paint composed of a solvent and organic pigment is applied, thereby electrically depositing a color coating on the anodic oxide coating. This layered structure enhances not only the anti-corrosiveness of the magnesium diaphragm, but also the decorative value of the diaphragm.
However, since the electrically deposited color coating for the decoration is thick, the magnesium diaphragm disclosed in Reference 2 is heavy and may fail to exhibit so good characteristics as desired.
As indicated above, since the magnesium diaphragm using the baking finish in Reference 1 and the magnesium diaphragm colored by the electrodeposition after the anodic oxidation in Reference 2 have the thick and heavy coating, the characteristics as the diaphragm might be impaired.